The present invention is concerned with a radio communication system and relates to an arrangement and to a correction method in connection with such communications systems.
Electronic compasses are used as an alternative to the well known magnetic compass, in order to determine and show the direction NORTH. Electronic compasses are used more in electronic positioning and navigational systems.
The U.S. Pat. No. 5,353,241 teaches an electronic compass system that includes, among other things, a flux-gate sensor that has an associated flux-gate drive circuit for sensing the direction of the terrestrial magnetic field.
A flux-gate magnetic-flux sensor includes two sensor coils positioned perpendicularly in relation to one another. Magnetic fields induce voltage across the coils. The aptitude of the voltage will depend on how the compass, and therewith the coils, are orientated in relation to the direction of the Earth""s magnetic field. This dependency is obtained, for instance, by virtue of the pair of mutually orthogonal sensor coils surrounding a ring-shaped core of magnetic material which is saturated and de-magnetized periodically. The sensor coils therewith generate periodically outward signals which represent the length of the component vectors on respective coordinate axes in a Cartesian coordinate reference system. The system coordinate axes are defined with respect to the compass and its angle of rotationxe2x80x94with regard to the direction of the magnetic North Pole. The result, in other words, the vector sum of the component vectors, therewith points to the magnetic North Pole, whereas the rotational angle of the coordinate system represents the angle through which the compass is rotated from the direction towards north, the vector of the Earth""s magnetic field. Theoretically, the vector sum of the two outward signals of an electronic compass should follow a circle with its center in the origin of the reference system when the compass, and therewith the coordinate system, is rotated through 360xc2x0, i.e. a full revolution. The circle is deformed, however, by disturbance fields.
These magnetic disturbance fields can be compensated for by a compensation circuit and a microprocessor, as described in U.S. Pat. No. 5,353,241.
The solution taught by the U.S. Pat. No. 4,482,255 uses a Hall-element as a magnetic flux sensor. It is also known to use magneto resistive sensors as magnetic-flux sensors in electronic compasses; see U.S. Pat. Nos. 4,640,016 and 5,600,611 in this respect.
In order to use the electronic compass, it must be calibrated. U.S. Pat. No. 5,165,269 describes, among other things, an arrangement and a method for calibrating the compass so that it will give a true reading.
The aforesaid electronic compasses do not, however, take the local direction from true north into account. As is known, there are different, wide deviations from true north at different geographical places. In certain places, these deviations are so great as to necessitate compensation.
A direction determining device is known from document WO,A1,9305474. A user is able to determine different directionsxe2x80x94the the direction to the starting point, to the target and to the North Pole with the aid of a direction indicator and different illuminated indicators. The device can be carried in the hand of a user and includes a flux-gate compass for determining bearings and a GPS-type radio navigational receiver (Global Positioning System that use geostationary satellites) for determining the longitude and latitude coordinates for each geographic position. A directional computer in the device fetches from the GPS-receiver latitudinal and longitudinal information relating to the current position of the user and his bearing relative to north from the flux-gate compass, while making correction for local deviations with the aid of a separate unit (213 in FIG. 2). This unit indicates the geographic North Pole.
A further device that includes both magnetic field sensors and GPS-receiver is described in document WO,A1,9748025. This device functions to determine the geographic north direction from the measured magnetic north direction, by correcting the local magnetic deviation place in questionxe2x80x94information that is stored in the device. One problem with this solution is that a large quantity of data is required to compensate for misreading of a compass unit if the unit is intended to be sold and used in different places in the world, which means that each individual compass unit must include at least one data store of large storage capacity.
Another problem is that it is difficult for an inexperienced user to use an electronic compass and GPS-system. The more complex the system the more training and regular use required for a user to become sufficiently clever to use the system without making mistakes. This can have serious consequences.
Each individual compass unit that includes a GPS-receiver must therefore include a data store of large storage capacity for storing misleading information in respect of each area or place in the world when such compass units are intended for sale and use anywhere on Earth. The costs of each compass unit increases as a result of this requirement of extra data capacity.
The invention provides a solution to the problem of providing an inexpensive and simple compass unit which has, nevertheless, automatic compensation for different deviations in the terrestrial magnetic field at different places in the world.
One object of the invention is to provide an inexpensive compass unit which, nevertheless, has automatic compensation for different deviations in the terrestrial magnetic field at different places in the world.
Another object of the present invention is to enable an inexperienced compass user to obtain a correct reading to the geographic North Pole in a simple manner.
In brief, the invention solves the aforesaid problems with a novel electronic compass unit, a novel method in respect of said compass unit, and a novel cellular radio communication system. An electronic compass unit constructed in accordance with the invention and located within the area covered by a radio communication system obtains, with the aid of a connected radio unit, information concerning the local magnetic deviation from a correct direction to the geographic North Pole from a central data storage unit in the system. Information concerning the local magnetic deviation is transferred from the central data storage unit to respective radio units via a downlink from a radio base station located close to the radio unit.
More specifically, the invention relates to a cellular radio communication system which includes radio base stations for radio communication with radio units over the air interface, said radio communication system being characterized in that it includes a data storage unit which is central to a plurality of radio base stations and from which information concerning the local magnetic deviation from the true direction of the geographic North Pole can be transferred to respective radio units from a base station located close to said radio unit, via a downlink.
The invention also relates to an electronic compass that includes an indicating unit which shows the direction towards the geographic North Pole, and an arithmetic unit or calculating unit for processing input data from a sensor which senses the Earth""s magnetic field, wherein the compass is characterized in that a radio unit for cellular radio communication is connected to said calculating unit. The radio unit is designed to receive and forward information concerning the local magnetic deviation as input data to the calculating unit, which, in turn, is designed to deliver a corrected input data signal to the indicating unit for indicating the direction to the geographic North Pole.
The invention also relates to a method of compensating automatically for magnetic deviation in an electronic compass, wherein the method includes the step of determining the direction of the Earth""s magnetic field on the basis of input data from said sensor. The inventive method is characterized by inputting data relating to the local magnetic deviation into the calculating unit via a radio unit in a cellular radio communication system, and by causing the calculating unit to calculate a compensated direction which is delivered to the indicating unit in further steps, to indicate the direction of the geographic North Pole.
One advantage is that large and expensive storage devices are not required for storing the misleading values corresponding to different Earth positions.
Another advantage is that the invention facilitates the use of this type of compass.
The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings.